Diabetes mellitus is a medical disorder characterized by persistent variable hyperglycemia (high blood sugar levels), resulting either from inadequate secretion of the hormone insulin, an inadequate response by the body's cells to insulin, or a combination of both of these factors. The most common forms of diabetes are type 1, type 2 and gestational diabetes.
Diabetes is a disease that causes many complications in human normal function. As a result, the amount of glucose in the blood increases while the cells are starved of energy. Over the years, high blood glucose damages nerves and blood vessels, leading to complications such as blindness, heart and kidney disease, nerve problems, gum infections, and amputation.
People with diabetes experiences symptoms such as feeling tired, losing weight, feeling hungry or thirsty, urinating frequently or having vision problems. Symptoms can appear after 10 years or more living with the disease.
This disease also represents the sixth-leading cause of death in USA. Prevention of diabetes-related complications can be accomplished through tight control of glucose levels in the blood. In the last decades many different glucose sensors have been developed. For example, existing glucose sensors and methods include near infrared spectroscopy, optical rotation, enzymatic assays, fluorescence detection, a glucose sensing contact lenses, a biochemical catalyzed reaction embodied as a Continuous Glucose Monitor System (CGMS), and a product called GLUCOWATCH®. However, none of these sensors is fully non-invasive.
The Continuous Glucose Monitoring System (Medtronic MiniMed, Northridge, Calif.) and the GLUCOWATCH® G2 Biographer (Cygnus, Inc., Redwood City, Calif.) are reviewed below. These two devices, each approved by the FDA, measure a signal that is reportedly proportional to blood glucose.
The first device for reading blood glucose levels continuously was approved by the U.S. Food and Drug Administration in June 1999. This device was the Continuous Glucose Monitor System (CGMS) manufactured by Medtronic MiniMed. The CGMS measures interstitial fluid glucose continuously. It calculates and stores a reading every five minutes over a 72-hour period. The CGMS does not provide the glucose results in real time, but rather downloads the readings after they have been collected, the way a 24-hour cardiac holter monitor provides information about cardiac rhythms after they have occurred.
The CGMS contains a wire with a supply of glucose oxidase at the tip, which is inserted subcutaneously into the anterior abdominal wall. This same enzyme is used in many portable blood glucose monitors. Glucose oxidase catalyses a biochemical reaction in the presence of glucose and Oxygen that transfers electrons to a receiving molecule and creates an electronic current, whose magnitude can be measured and converted into a glucose concentration. After 72 hours of measurements, the device is removed and plugged into a docking station to download its readings.
The docking station can be connected to a computer that contains dedicated software for use with the system. The computer will then print out a graph of the three days' blood glucose readings. The CGMS must be recalibrated with a fingerstick blood glucose reading at least four times per day.
The GLUCOWATCH G2 Biographer is worn attached to the wrist, forearm or arm, like a wristwatch. The device measures blood glucose in real time with new readings displayed every 10 minutes and a trend arrow to indicate whether the blood glucose level is rising or falling. Both of these continuous monitors measure the glucose concentration within interstitial fluid glucose.
The GLUCOWATCH® G2 Biographer contains two electrodes, which pull salt from the skin, the salt carries water, and the water carries glucose. The glucose solution, which is a form of diluted interstitial fluid, is collected into a pad called an Autosensor. The Autosensor component of the GLUCOWATCH® G2 Biographer must be replaced after 13 hours of readings are made. The glucose concentration is determined by an electrochemical reaction in the Autosensor. A calculation of the blood glucose level is made every ten minutes. A glucose reading is displayed on the watch face every twenty minutes.
As described above, both the CGMS and GLUCOWATCH® are invasive. Even a contact lens for monitoring glucose is somewhat invasive, besides being uncomfortable. What is needed is a portable, low cost, truly noninvasive glucose sensor that provides high sensitivity and no discomfort to the patient.